The offer, which newspapers said was made on Sunday before Cardiff lost 1-0 to Tottenham Hotspur at White Hart Lane, was withdrawn once Tan was informed it was illegal under the league's player remuneration rules.
"We have asked the club for their observations," a league spokesman said.
Newspapers quoted a spokesman for Tan as saying the offer had been a mistake.
"We didn't realise it was against regulations," he said. "Vincent Tan has now rescinded the offer."
Cardiff are 19th in the 20 team league, one point clear of bottom side Fulham and three adrift of 17th-placed West Bromwich Albion.
League rules state that full details of a player's remuneration, including all benefits he is entitled to in cash or in kind, should be set out in a contract that must be strictly adhered to.
($1 = 0.5982 British pounds)
- Sports & Recreation
- Vincent Tan
- Tottenham Hotspur