The OP isn't asking for reasons why his request cannot possibly succeed; he asked for information that he could use to persuade his supervisor to agree to his request.
"If you don't ask, you don't get." That precept has served me very well over the years. More often than not I have gotten some or all of what I wanted simply by asking for it, sometimes in situations that most people would say it was not possible to do so.
Some years back I was involved in an intellectual property dispute with a large, well-known corporation. The impending lawsuit could have cost me in excess of $100,000 to litigate. Frankly, I couldn't afford it and I would have had to let my claim go.
So I did the unthinkable: I contacted the in-house counsel for the company and explained the situation to her. I asked her if there was some way that we might resolve the dispute that would work for everybody. She responded that she could not think of how we could accomplish that goal; I ended the call by asking her to let me know if anything changed and thanking her for her time.
I figured that was the end of it. But a week later I got a call from the attorney. She told me that she had been thinking about the situation after our conversation and that she had come up with a possible solution. She thought that she would be able to convince her boss to go along with the plan.
I put her in touch with my attorney and over the course of a few days we negotiated the details. To make a long story short, in the end I got what I wanted and the company felt that its interests were protected. My total cost: $2000.
After it was over my attorney called me and told me that she had never seen anything like it. The company had absolutely no reason to expend any effort to negotiate with me; it essentially held all the cards. But the in-house counsel told my attorney that my call had convinced her that I was a straight-shooter and that I wasn't trying to scam the company. She was so impressed with what I had said to her that she took it upon herself to come up with a reasonable compromise and then went to the trouble of convincing her boss to agree to the deal.
The moral of the story: don't assume anything when approaching a problem. Every situation and the people involved is unique, so start with the idea that you can succeed. In other words, don't give up before you start. Only fold your cards when you know you are beat.
