How does it raise the possibility of fabrication? If I pay for research to be conducted, my selfish use of the results doesn't change the results of the research.
Well, first, there's the tendency for oppo researchers to produce research that their clients want. It's an exercise in confirmation bias right? Fusion GPS is not going to put exculpatory information in the dossier. It was explicitly created to make Trump look bad, since Fusion GPS knew who was paying for the research and (presumably) how it would be used. So that's the cherry-picking problem.
Second, by hiding its provenance (and explicitly denying responsibility for funding it), the Clinton posse made it seem more credible. The FBI probably would have been more skeptical if it came from the Clinton campaign directly. Since nobody seemed to know where it came from (the FBI, including Comey, denied such knowledge, but I suppose they could have been lying too), the Clintonites must have passed it on by way of a cut-out.
Third, if I were in the Clinton camp, I would relish the thought of making allegations and then using the oppo research to lay out bread crumbs for the FBI/CIA/NSA to find and get them ever more suspicious. One of Vogel's twitter interlocutors lays out a plausible scenario (although I would eliminate Step 3 below):
Step 1) Clinton campaign pays for Trump dossier based on info from Russian gov't sources
Step 2) Dossier is shopped in DC by Fusion GPS
Step 3) Trump dossier is published by Buzzfeed, taking it public
Step 4) FBI uses Trump dossier to apply for FISA warrants
Step 5) FISA warrants are granted based on disinformation from Russian sources
Step 6) Trump campaign chairman/others are wiretapped
Step 7) Trump wins election, FBI offers to pay Steele to continue gathering intelligence on Trump
Step 8) Steele outed, FBI backs out
Step 9) 10 months of DNC Media/Dems frantically trying to find evidence of the Trump campaign colluding with Russians to influence election
Step 10) Media finally gets around to admitting Steps 1-8, everybody realizes they've been wasting their time.
RUSSIAN RESET!