Security issues when PI in DMZ

Dear All,

I have a scenario (without ESR) where I need to process the Payment file(text file) to bank. However, as PI system is in DMZ, and customer's infrastructure team is not willing to open the SFTP port(22) from PI to inside their local network where ECC has been placed. consequently I am not able to connect the ECC system from PI using SFTP adapter, because they are feeling security issue here, as if port is open from PI to ECC it can make a way to hack the other systems in their local network.

Please advise how we are dealing with such kind of scenario.

Thanks,

Farhan