That was one of the many dubious allegations doing the rounds, certainly. There were two major problems with it. Firstly, the supposed "dropping of sanctions" a month after Trump took power didn't actually happen. The thing people claimed was dropping the sanctions was actually a small technical change that merely allowed US exporters to pay fees required to export goods that the sanctions weren't intended to block: http://www.snopes.com/trump-sanctions-russias-intelligence-a...

Secondly, the claim that the Russian government offered Trump a share of Rosneft came from a really dodgy dossier. Supporters of the dossier touted the fact that it correctly predicted the percentage of Rosneft share sold as proof the source had inside knowledge, but it was in fact public knowledge that the Russian government wanted to sell that percent of Rosneft and had been reported in Bloomberg a few weeks before the supposed source mentioned it. So it's all too plausible that the source was merely combining public information with what the person paying them wanted to hear.