There was a few sources. I read it in the Guardian but it's saying i've read 12 articles this year now so can't access it. It was also quoted in a Talksport interview - but not sure what source they were using.
There's also other sources that are saying Barca owe nothing, as how the deferred wages/contract was structured means that if he leaves, the money isn't technically owed anymore. So the 3.5m offer would be a literally a peace offering.
Peicing it all together, we see that it's certainly the money thats the problem - and Barca either want to pay installments or a minimal severance package on what he is morally owed.
De Jong should not back down, and we shouldn't bail Barca out of the situation. They'll break eventually and either keep him on his contract (to be paid in full) or pay him off and get him out (most likely).