Paul planned to bring some Macedonians with him to Corinth. The "if" does not imply doubt about this possibility in the Greek text. The first class condition in the Greek text describes a condition Paul assumed to be true for the sake of the argument. In this case we could translate the Greek word for "if" as "when" (cf. John 12:32; 1 John 2:28). Nevertheless there was a possibility that Paul and his Macedonian companions might find the Corinthians unprepared when they arrived. Paul evidently mentioned his intention as an added incentive for the Corinthians to complete their collection. [source][source][source]
"There were two situations Paul wished to avoid. One was that his repeated and confident boast to the Macedonians about the Corinthians" "eagerness to help" ( 2 Corinthians 9:2) and their expected "readiness" on his arrival should turn out to be without foundation ( 2 Corinthians 9:3). The other was that when the delegates of the Macedonian churches (not to be confused with the two companions of Titus) arrived at Corinth with Paul on his forthcoming visit ( 2 Corinthians 12:14; 2 Corinthians 13:1-2), the Corinthians would be still unprepared and this would lead to his acute embarrassment-not to mention that of the Corinthians themselves ( 2 Corinthians 9:4)." [1][source]
"He is not afraid that they will refuse to give, but he is afraid that they may be dilatory for want of organization. It will produce a bad impression if the money is not ready when it is wanted. He carefully limits his anxiety to "this particular."" [2][source]