So, I am trying to pursue an IFA and I have maxed my 18mil budget. Why won't it let me offer a big league contact for more than 327k? My last offer was 18mil plus minor league contract with invite. Oh, and I don't have more money to transfer. Can I offer anything better?

Because if owners could use player salary for IFA, there would be more incentive to strip the ML roster of anyone making decent money when rebuilding. Top IFA would get even bigger contracts than they do now, and to compete for top IFA teams would have to dump salary. If you've maxed out your Prospects budget and have no surplus to transfer, you're not going to get a good IFA. As it should be.

Max salary you can offer an Int FA is $327K / season for 3 seasons.  If there's a limit on the bonus, I've never seen it.

Like many things in HBD, the answer is "because that's how it is".

Sometimes there are good reason. Sometimes not. Sometimes people debate if the reasons are good or bad.

There's a suggestion section here you can use if you think something should be changed or improved.