Green declared earlier this week that the Ibrox club had paid all of its outstanding football debts to Scottish clubs – a condition of the transfer of SFA membership.
But United responded to that by announcing that they had still to receive a sum of around £31,000 from a Scottish Cup game with Gers last season at Ibrox.
But Rangers supremo Green yesterday released details of a letter sent by the SPL to Rangers on May 18 – in which the governing body agrees to reimburse United.
And league chiefs are currently speaking to SFA Director of Football Governance Andrew McKinlay in an attempt to clarify their position.
An SPL spokesman said last night: "We are now discussing this matter. We will be making no further comment at this stage."
Green, however, is in no doubt that it's the SPL and not Rangers who will have to cough up.
He said: "As previously stated, Rangers Football Club can confirm it has paid all outstanding debts to Scottish football clubs, in addition to agreeing to waive all claims to unpaid fees accrued last season, as part of the Five Party Agreement which was a condition of Scottish FA membership.
"A letter from the SPL to the club – dated May 18, 2012 – stated: 'The board decided to accede to the application of Dundee United and accordingly, the sum will be withheld from the next sum payable by the SPL Limited to Rangers and the sum will be paid by the SPL Limited to Dundee United.'
"Why the SPL have not paid Dundee United the outstanding sum as previously agreed is a question that they need to answer. We wrote to Dundee Utd on Monday explaining the SPL had previously confirmed they would pay it.
"I spoke to Stewart Regan and his solicitor this morning and provided them with copies of all relevant documentation."
The SFA are believed to have honoured their side of the agreement.
Green's new company agreed to pay all football debts as part of the arrangement which saw Rangers' membership of the SFA transferred. He also agreed to waive the £2.55million Rangers were due for finishing runners-up last term.