David Luiz helped Chelsea negotiate a good deal for his transfer from PSG

Good deal and bad deal are relative terms, of course, but considering that the initial rumor amount for David Luiz's transfer fee was £32m, which was then supposedly rejected before Chelsea came back with an improved £38m offer, if Chelsea did indeed manage to sign David Luiz for "just" £30m, then this deal is looking better by the moment.  Not that it really needs that incentive; David Luiz returning to Chelsea is already glorious enough.

PSG refused to sell David Luiz until he said yesterday he wanted Chelsea move. Fee £30m. PSG also turned down Chelsea offer for Marquinhos

— Kaveh Solhekol (@SkyKaveh) August 31, 2016

Those involved in David Luiz PSG to Chelsea negotiations felt the £30m deal had little, if any, chance of success until late this morning

— Kaveh Solhekol (@SkyKaveh) August 31, 2016

When Luiz heard yesterday PSG refusing to sell him to Chelsea, he spoke to PSG president & sporting director & convinced them to let him go

— Kaveh Solhekol (@SkyKaveh) August 31, 2016

So apparently the key factor was David Luiz saying that he wanted to leave, though there were rumors yesterday that he wouldn't force a move.  That said, there's a difference between wanting to leave and forcing a move, as we also saw with Diego Costa earlier this summer.

Interesting side note about the younger Brazilian defender in there.  Chelsea have had a long-standing interest in Marquinhos, getting linked last summer as well.  But for now, David Luiz will do just great.