If Professor McGonagall writes a letter to Quality Quidditch Supplies explaining the situation, I imagine they won't deliberately conceal anything.
If the Firebolt really was a Christmas gift sent to Potter by Sirius Black, the sender certainly couldn't have swaggered openly into Quality Quidditch Supplies; they would have had to order it by mail.
Under normal circumstances, as long as the Firebolt passed through no other hands and was sent directly to Potter by Quality Quidditch Supplies, there shouldn't be any other problems with it.
Chapter 678