##You make agreements when on good terms before things can go bad.

You prefer a confirmation of your payment, so that no misconceptions can arrise in the future. This way, both of you have something to fall back on as it is on paper, so that your friendship does not have to suffer.  
You're on good terms now, best to come up with something both of you agree on. When you get into an argument, you often loose the perspective of the other, and reading back your original agreements can save you a lot of energy and possible friendships.

Apart from that, if you give your transaction/cheque a description which is obviously relating this subject, you can always check back your bank records.