The alternative is to keep him on the books, pay him, and be no better off in the process.
That's a strange position. I'm not especially interested in his destination. In the vast majority of cases, St Mirren have absolutely no influence over who other clubs sign. That's as it should be, and we don't (or certainly shouldn't) lose any sleep over it. I'm happy to treat this as just another instance of that.
This is simply another football transaction. We are well rid, frankly. How he performs at Dundee is their concern.