There are some ways to approximate this, but as seidel pointed out, there's no way to "clean" the data 100% effectively unless the reads happen to be tagged for either condition.
Off the top of my head, there are two approaches you could investigate further:
if tissue and graft have different genetic signatures, e.g. come from different persons or even different organisms, then you can distinguish those reads that fall into regions covering loci that differ genetically
since you have a sample that represents the tissue alone, you could try methods that attempt deconvolution of bulk RNA-seq signal (just google that, you'll find plenty of packages)