Let's say for argument's sake that Hull have, as you say, increased their offer from 750k to 1m with a 10k pw salary and Saints original offer was 1.5m with, say, a £5k pw salary, Saints could now say to Swindon: "Ok, we'll now offer you 1.25m" and use the 'saved' 250k to pay him a higher weekly wage than Hull.....
Same amount of money we were prepared to throw at the deal, just distributed differently.
All that assumes the figures quoted are correct of course....