I would offer to pay before you get an outfit - afterall if she is hard up she may need to say yes! If you just hand over an outfit then she may feel really peeved that she has had to pay for a new pair and, instead of helping with the cost, she now has an outfit that she may not want/like/need!
Imagine the post on here, "After playdate my DD's shoes were lost, the Mum says she has looked and so I have had to buy a new pair - I am so hard up at the moment too . Then, today, the Mum shows up with an outfit from [insert shop name] which is a lovely gesture but I could really have done with the money towards the shoes, what do I do now!"