Your stock is one unit & there are 2 urgent orders, one from an existing customer and the other from a new customer. who will you supply & why?

First come first served.

If the question was about who to serve first an existing customer or a prospect, I will choose for the existing customer as there is certainty. But as you say existing customer or new customer, both are customers, so  I will try to serve the new customer with permission of the existing one in order to build trust,. 

I would take an estimated risk based on the existing customer loyality !

Delivery priority will be given to the new one.

That's simply my vision. 

I am not in the field of sales/marketing, but I do understand that when doing business you have to treat your customers as kings, so you must respect your existing customers, but at the same time you can't ignore your new customers also as it said 'first impression is the last impression'. So a perfect solution for this problem should be that you arrange additional stocks from where-ever you can, if need be you purchase at higher cost and bear some loss for this transaction, but make sure you are satisfying the requirements of both customers.

Existing Customer.

Selling to existing customers is quite different to approaching new prospects. You don't need to establish your reputation, skills or the quality of your products. The customer is already convinced. The fact that you're telling them about other useful products or services shows that you understand their needs and care about their satisfaction.

To me, that situation is very impossible to happen and it should never happen. That is bad for business. Situation like this, I would get stocks from the nearest manufacturer or distributor and supply both. I'd rather pay the expenses than loose the opportunity of selling. 

If the business is B2B : The same will be given to My existing customer. (My existing customer is always precious to me and  I will not a give a single chance for any potential threat to his business line through my hands and don't want to entertain a customer at his expense)

If the business is B2C: I will give preference to the new customer. (only when the existing customer is approaching me more on the basis of loyalty and I can convince and compensate him subsequently based on the personal faith n him)

Of course, Existing customer. From how long , how many years or months,  he is doing businesss with you. How many times he has given 'Bread and butter' to you.

New customer will be vanish in the crowd tomorrow. If you keep good relation with  your exiting one, he will give you hundreds new customer.

Its always about you! who are you? what do you want, and whats your strategy ?- You will serve the new customer if: 1- you are a big name in the market

2- your brand has no substitutes 

3- you are solely controlling the market4- if your brand quality has no competition

you will serve the existing customer if: 1- you are a new name in the market 2- if you have a saturated rate of sales3- If demand in low in the market4- if you are having lots of cometition5- if you are implementing a defensive strategy

Existing customer. We should never lose our existing customer.

There is absolutely no way that you can jeopardize the relationship of an existing customer whether its a B2B or B2c. Its better to retain a partner by addressing his requirement than to lose out on a relationship and spend more on acquiring new leads.

Communication is the key wrt the new customer, the idea is to be as honest and ensure that the unit is provided in the quickest span of time. Or very politely agree to regret to have missed the opportunity to service that lead. The last thing a new customer needs is a false promise!

ANSWER: I will serve the existing customer

REASON: It is always fruitful to serve to the existing customer because existing customer is already doing business with us, and it means that he is satisfied (or may be delighted) with our market offering, so he is offering more business to us. So, it is more logical to serve the existing customer. Saying NO to existing customer means giving him an opportunity to meet our competitors, which will be a major failure.

HOWEVER, I will also communicate with new customer, take keen interest in him, and offer them the best date when we can generate the new market offering. Meanwhile, I will try to be engaged with new customer, and explain the special features to him, and if possible, I will offer him some special benefit (as per my company policy, if so exists), so that new customer remains interested even if he has to wait for our market offering.