No he doesn't need Congressional Approval to build it, it's already an existing program account on the books of Executive Branch Agencies (primarily DHS and DoD) and there is no need to "take out loans". The Executive Branch has budgetary transfer and re-programming authority to move funds (within limits specified by statute) from program account(s) to other program account(s) within the same Executive Branch Agency or across Agencies, in other words take money out of X, Y and Z program accounts and move it N program account to fund N program and then "re-program" X, Y and Z program accounts to reflect the alterations (i.e. modify timelines, standards, payments, etc..,).
It's pretty routine stuff for the Agencies of the Executive.
Of course blatantly abusing that authority invites retribution by the keepers of the purse strings when it comes around to appropriations time.