Dave Robins from Swanley, Kent, leased non-emergency ambulances to Coperforma, which was stripped of its Sussex patient transport contract on Tuesday.
He began providing them in July after two other suppliers went bust, but has said he has only received one payment.
Coperforma said all contractors were paid in full for services received.
"In line with our contractual policy we do not discuss with third parties details of the relationship we have with individual transport service providers," a spokesman said.
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Mr Robins, of UK Ambulance Sales, said he lost almost Â£200,000 in the summer after two patient transport companies in Sussex went bust.
Coperforma stepped in to keep the service running and, he said, did a deal with him to lease ambulances for six months from 1 July.
"They made payments for 1 July, supposed to be a six-month basis, [and] haven't made payment since.
"Invoiced them for the full amount. Nothing at all, so I feel quite aggrieved about what's gone on," Mr Robins said.
Not for the first time Coperforma are having to deal with disgruntled contractors.
We saw it with VM Langfords, Docklands Medical Services, Dedicated Care in Eastbourne and now UK Ambulance Sales.
As the "managed" handover between Coperforma and South Central Ambulance Service begins, so people within the industry are telling me that confidence in the service and in Coperforma is fragile.
Indeed, "fragile" is how the boss of the Clinical Commissioning Group, Wendy Carberry, described the entire Sussex Patient Transport Service when I interviewed her this week.
It might be  that fragility leads to more problems in the months to come.
The spokesman for Coperforma said: "I can categorically state that our payments to our contractors are up to date for service received to support the Sussex patient transport service.
"I can also confirm that we are in different stages of negotiations with a small number of providers over invoice queries which we are trying to resolve as speedily as possible."
Coperforma is due to hand over its patient transport service for Sussex in a "managed transition" to South Central Ambulance Service by next April.
A businessman who supplied ambulances to an NHS patient transfer service says he was left with "thousands of pounds of debt" because he was not paid.