And yet Paul circumcised Timothy to spread the gospel. Was Paul 'sinning' when he did that? Does it occur to you that it may have been a sin NOT to circumcise Timothy in that situation?
Likewise, could it not also be a sin NOT to tithe if God calls a believer to do so to spread the gospel?
He who has ears let him hear.
No, because Paul specifically refused to have Titus circumcised and the entire issue at the Counsel of Jerusalem was whether gentile Christians would have to follow the law, particularly be circumcised. Paul won - the outcome of the counsel of Jerusalem was that gentile Christians were not bound by the law and circumcision was not required. Please read the book of Galatians, it covers all this.
And no, Timothy was not sinning when he was circumcised. Romans 14 deals with this and if someone not bound by the law chooses to follow it they are not sinning. Paul just calls them weaker Christians. Finally, no it is no a sin not to tithe. Spreading the Gospel has nothing to do with this issue. Whether or not one chooses to tithe has nothing to do with their effectiveness in spreading the Gospel.