* 밑줄 친 부분 중 어법상 옳지 않은 것은?

a man who (1) shoplifted from the woolworth’s store in shanton in 1952 recently sent the shop an anonymous letter of apology. in it, he said, “i (2) have been guilt-ridden all these days.” the item he (3) stole was a two dollar toy. he enclosed a money order (4) paid back the two dollars with interest.

어법상 틀린 것이 4번이라고 해서 paid back를 paying back으로 고쳐야 한다고 하는데

to pay back 이라고 하면안 되는 것인가요??