there is no loss in savings interest. OP's sep statement is for 26 aug to 25 sep, so doesnt matter if he pays on the 1 or 10 or 15 or 20 of sep. for any credit card payments, if there are any loss in interests, it will be counted by month(s), not days, so in your case of 100k, one is losing out on 400+ pre tax, a pretty significant sum

his question is whether the sep 15 cut off is by transaction or statement. i.e. even if he paid on 1 sep, would he have gotten the 4% because his statement closes on 25 sep, which is after the 15 sep cut off. to answer RobUAIntl , the cutoff should be by transaction. i dont have the card, but based on the brochure i see online, it said "the following rewards program updates will take effect 15 sep". so that cant be true for some transactions if cutoff is by statement closing date. it will also be highly unfair to people whose statement closes on and after 15 sep, not that i think USB truly care about fairness except that there would be a lot of unhappy customers and backlash to deal with, so it'sunlikely they do something like that. but of course, the ideal is to have it down in writing from USB

also note that is it said take effect 15 sep, so the transaction have to complete by 14 sep

eta: this is also somewhat false depending on how one intepret smaller. if there are any loss in interests, the absolute amount is smaller if there are lesser taxes, but its proportion to the loss of 2% cashback stays the same

100k taxes:
assuming net tax rate of 40% and high yields ir of 5% (which is what FFI used)
loss in interest is $250
2% loss in cashback is $2000
interest is 12.5% of cashback

40k taxes
assuming net tax rate of 40% and high yields ir of 5% (which is what FFI used)
loss in interest is $100
2% loss in cashback is $800
interest is again 12.5% of cashback