Interest rates

This may be a silly question but I'm doing question 84 in bpp revision guide. I won't write out the question but I'm trying to calculate the saving a company makes on interest. In the question if a factor pays advance for receivables the interest rate on the advance would be 2% higher then the 7% the company currently pays on overdrafts. The answer calculates 2% of the advance and uses it as as cost rather then a saving. I can't understand why it would cost us 2% on money we are receiving from the factor in advance. Surely we are saving 7% that we would have had to pay if we didn't have the money in advance. I'm obviously missing something. Thank you in advance.