I'd view it much more in the realm of Fergie/Gill saying they want a player and what they think he's worth, open informal discussions with the other club, loosely discussing what they'll pay having discussed it with the Glaziers, agreeing a fee with the other club and then having the Glaziers sign off on it.
As for the rest, I don't know what drives these decisions, or why we stop at a certain point. If I had to guess it's because we believe that if we makes ourselves and easy mark for other clubs they'll continue to make us pay more and more.
But I'm absolutely sure it - at least in part - relates to where the profits we're making go if we don't spend it. It really just depends on how much Fergie pushes it, in my uneducated opinion. Because he's a parsimonious sort he probably makes it easy for them by not wanting to push the boat out. Though his refusal probably has less to do with wanting to service the debt than it does an overriding view on inherent values in the market.