lending to a friend is a very tricky issue

since if he/she fails to pay as agreed your friendship is ruined. My advice would be, since he/she already knows you have the funds, just lend him 1/4(quarter) of what he/she is borrowing and let him/her know that the rest of the funds are already commited. Also info him/her that you will need the cash that u have lend him/her in a few weeks time so that he does not become comfortable with your cash. repayment should be at the earliest oppurtunity possible.
in most cases avoid lending to friends and relatives unless in exceptional cases