Should I use Goal Seek, a function, or a macro?

Hi Excel Gurus! I have a tough one, and was wondering if anyone could offer advice. My problem is this:

I have a column of invoice values. I know each value and it comes out to a total of about $40,000. Now I have received payment for only about $16000, but I don't know which I have received payment on, and which still need to be collected. Can excel help sort these values out?

Was wondering if this is something Goal Seek could solve, should I write a macro, or is there a funcion/formula that can handle this.

Any help would be greatly appreciated.

Yes, I know I should keep better records... but it's not for me, it's for my friend, I swear! The problem is currently being solved by trial and error until the values work out.

--Hashi

This is a problem that is probably best solved using Solver. It's been discussed several times. Basically you have a columns of 1's and 0's (representing which haven't been paid, and which have), then a simple sumproduct formula to add up the total remaining (or the total paid, whichever way you want to do it). Then use Solver, constraining the 1's and 0's to binary so they stay 1's and 0's.

The main concern here would be knowing if there is only one possible solution. If there are two possible solutions, you'd better know some of which invoices have been paid. Wouldn't want to charge somebody who has already paid.