It is understood Prost's severance deal with Williams-Renault involved a payment of dollars 8m ( pounds 5.4m) to the four-times world champion. If Prost returns, that money must be repaid and some, or all of it, will be go to Senna, his successor at Williams.
Senna, asked if he would welcome the appearance of Prost this coming season, replied: 'All I will say is that it would do a lot of good for my bank account.'
Williams may have been unable to meet Senna's financial demands, but satisfied him with this contingency clause. Last year Senna earned dollars 1m for each of his 16 races with McLaren.
Frank Williams, the team principal of Williams, confirmed: 'Prost would have to contact us before signing for another team. He left after only one year of his two-year contract with us. It could be embarrassing for all concerned, especially Alain.'
Prost is due to test the first McLaren-Peugeot early next month, but he is still believed to be interested in taking control at Ligier-Renault. Peugeot officials acknowledge, however, that he may use any offer to strengthen his bargaining position.
However, Prost insists he is seriously considering competing again in response to a poll of motor-racing fans in France. Some 54 per cent of voters thought he should carry on and would not lose face by doing so.
Among those still hoping for a drive with McLaren are the Britons, Martin Brundle and Derek Warwick. Another Briton, Johnny Herbert, could also be in contention.
Benetton-Ford's new driver, JJ Lehto, recovering from a six-hour operation on his fractured vertebra, is expected to resume driving in eight weeks. Since the season starts on 27 March, it seems almost certain that he will miss at least one race.