I didn't make the agreement. It's been widely reported on.
The US treasury said that an investigation into government officials and gang leaders revealed the secret negotiations. Luna [chief of the Salvadoran penal system and vice-minister of justice and public security] and Marroquin [chairman of the Social Fabric Reconstruction Unit] allegedly “led, facilitated and organized a number of secret meetings involving incarcerated gang leaders, in which known gang members were allowed to enter the prison facilities and meet with senior gang leadership”.
In addition to financial benefits for the gang members, incarcerated leaders received special treatment in the prisons, including access to mobile phones and sex workers. It said Luna also negotiated support from MS-13 and Barrio 18 gangs for Bukele’s national quarantine during the Covid-19 pandemic.
It sounds like the gang leaders who negotiated with Bukele's regime are incarcerated themselves. And it's not just prisoners in CECOT who are getting better conditions. It's incarcerated gang members all over the country.
The US government, like all governments, is comprised of individual people with varying motivations. The person asking for his return is not part of the executive branch, which is what's paying to keep people in CECOT.
A sitting Senator who is a member of the minority opposition party is asking for him to be returned. The US administration is paying the Salvadoran government to keep him there. The VP of El Salvador told Senator Van Hollen that if the US embassy asked for him to be released they would release him.