Andreas Grassl was found in a soaking wet suit
The patient at the centre of the "Piano Man" row should be forced to return to the UK to answer claims his condition was a hoax, the local MP has said.
Andreas Grassl was cared for at Little Brook Hospital, Dartford, Kent, until he flew home to Germany last month.
Media reports have since cast doubt on his condition.
Dartford MP Howard Stoate said the trust should get its care costs back if he was lying. The hospital dismissed this, saying Mr Grassl needed care.
Dr Stoate, a GP who sits on the government's Health Select Committee, said: "According to news reports I have heard there is real concern that he may not have been telling us the truth.
"I believe he should be brought back to this country to answer these charges so the public can be certain that the money they have invested in him is money well spent and not money that has been defrauded.
"If he's used this money in a fraudulent way then he's denied that treatment to other patients."
A statement released by West Kent NHS and Social Care Trust read: "The trust is not looking to recoup costs from the patient dubbed Piano Man.
German television reports said this was where Piano Man lived
"The patient not only needed to be in our care but is also a citizen of a country with which the UK has a reciprocal healthcare arrangement."
The trust said it could not go into detail about why Mr Grassl had needed to be in care, but his lawyer has said he was suffering from psychosis.
According to health experts, people who experience psychotic episodes lose contact with reality and often suffer hallucinations and delusions.
Mr Grassl, 20, from Prosdorf, Germany, was found wandering on a beach in Kent in April.
He did not speak to health workers until 19 August, but reportedly stunned them with his brilliant piano playing.
After his return to Germany, lawyer Christian Baumann said his client had been suffering from severe depression.
"It wasn't intended as an act of deliberate deception. He had a psychotic illness."
Dr Stoate said Little Brook staff had worked tirelessly on the case.
"He had extremely sympathetic treatment and a huge amount of time and effort was put into this man to try to establish his identity, to try to help his possibly psychiatric condition.
"If he's genuine, that's absolutely fine, he deserves to have that treatment.
"I don't wish to go down this route in particular but I do believe that significant public money has been expended."
Dr Stoate estimated that between £40-£50,000 might have been spent on Mr Grassl's care.
A spokesman for the NHS trust told the BBC that "an average cost for a patient, per night, in that type of environment is about £430". Mr Grassl was in care for 134 days.