However, in return for the Contractor’s approach, the Subcontractor has unfortunately failed to fulfill his obligations under the Subcontract Agreement...
First, I removed the extra "this" from the sentence. "Unfortunately" is placed awkwardly here, and it makes the sentence look like it has a comma splice in it. Since it's being used as an adverb, I moved it closer to "failed," the verb it describes. Also, it's not proper to put a semicolon after "However," because "however" is not a complete thought (unless it was meant to be exclaimed as an interjection: "However!" I would probably avoid this in a written piece though).
"In return for" isn't really the correct turn of phrase here. It implies that something was traded or exchanged. According to the story, that's exactly the opposite of what happened (the Subcontractor gave nothing in return). My instinct is to use "despite" in its place:
However, despite the Contractor’s approach, the Subcontractor has unfortunately failed to fulfill his obligations under the Subcontract Agreement...
The context here is that even though the Contractor has been generous and supplied the funds in advance, the Subcontractor did not fulfill his obligations; this is unexpected given the Contractor's terms. "Despite the..." is a good prepositional phrase to use here, since it describes that unexpected outcome.