OP, like many women, you seem to find it hard it hard to enforce boundaries and to say no.
Perhaps have a think about why this is so. Then practise saying no. When learning Assertiveness Skills, it is best to practice in situations with less emotional impact. Taking easy steps to becoming a woman who can say no in all situations.
Actually practising the words out loud beforehand can be helpful. Maybe with a different friend or family member. If that's not possible do it in front of a mirror. Get your mouth and brain used to the word.
With your friend, if you cannot do it face to face, a text could be more comfortable.
Don't just send it back without warning her. That's not fair. Tell her you need the money or you will send it back. Give her a deadline. A short deadline. After that use the broken record technique and repeat your needs rather than getting drawn into debate. If she argues back, asks for it anyway, asks for more time etc.
Lying and being fuzzy will not stop this pattern of behaviour. Being clear about your needs whilst being respectful of yours will be effective. If this is a real friendship of value, she will respect you and it will be a better friendship for that. You won't be silently resenting her and she won't be looking on you as a piggy bank.
A good book on the subject is A Woman In Your Own Right by Anne Dickson.