Ipswich have reported Swansea to the Premier League and asked for a transfer embargo to be placed on the top-flight newcomers for alleged non-payment of invoices.
Club chief executive Simon Clegg claims the Swans have reneged on a deal relating to the payment of Tamas Priskin's wages while he was on loan last season and subsequent add-ons as a result of their promotion.
"I am hugely disappointed a legally-binding agreement reached in good faith by both clubs as part of Swansea's final push for promotion is now being disputed," Clegg told itfc.co.uk.
Ipswich claim the loan agreement made provision for Swansea to pay for Priskin's wages up to May 31 even though he was injured on April 22.
They also say the contract provided for "cumulative payments for each stage of the play-off competition".
"I am genuinely delighted for Swansea and their fans but the attempted renegotiation and deliberate mis-interpretation of agreed terms by their chairman (Huw Jenkins) is not fitting for a club which has just secured the ultimate prize in Championship football and its £90million windfall.
"Tamas arrived in Swansea fit and became injured while under their care - that's football but contracts have to be honoured and I have absolutely no interest in renegotiating this agreement.
"As a result of Swansea refusing to pay our invoices I have reported them to the Premier League, sought an immediate transfer embargo and am now seeking full payment of all outstanding sums with the agreed contractual interest rate of five per cent above base."
Neither Swansea nor the Premier League were immediately available for comment.