The Daily Star says the 25-year-old Belgium star is understood to have waived a £4m bonus by handing in a transfer request.
And that led to reports that United were able to get their man for just £23.5m - the same amount as a previous buy-out clause that expired at the end of July.
But Everton maintain the transfer is worth £27.5m to them, with all the cash coming from the Premier League champions.
Martinez said: "Everything was done on our terms and that is what is important. They met the valuation - that is what happened.
"He had a buyout clause but that expired on July 31 and from that point on, the valuation was completely different and he had a huge premium on top.
"Everything was on our terms and it was about making our squad stronger and it left us in a good position."
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