I don't know how behind the scenes it is. I thought it was fairly obvioius they had reached "un acuerdo" when he returned to the fold in 2019. After all, he was her boy before he went indy and started his own party.
Massa joined the Justicialist Party back in 1990, he was director of ANSES, and then intendente of Tigre, and he was Cristina's Jefe de Gabinete in 2008-2009. It wasn't until 2013 that he jumped ship and founded that Frente Renovador. So him coming home to mama is hardly a surprise and he was richly rewarded by being made President of the Chamber of Deputies, rough equivalent of Speaker of the House in US terms.
Massa is a Beta with Alpha ambitions, but his miembro masculino just isn't quite big enough.