One council has told BBC Wales it intends using £20m of its cash before any reorganisation.
Carmarthenshire's head of resources said he wanted to see the council's money spent in the authority area.
The Welsh government said councils had to maintain reserves and it would act to halt "irresponsible spending".
But Carmarthenshire executive board member, councillor David Jenkins, said money should be spent on boosting the area's economy before any mergers.
"This is common sense. The question we're asking is what will happen to Carmarthenshire's money if we have to merge with Pembrokeshire and Ceredigion," he told BBC Radio Cymru's Post Cyntaf programme.
Although plans have yet to be finalised, he said the intention was to spend about £20m of £72m earmarked reserves on an "anti-austerity package".
He said there was money within the reserves "where the initial purpose has come to an end" which is the cash which would be used to improve the area's economy.
However, not every local authority agreed with Carmarthenshire's approach.
Ceredigion's leader, Ellen ap Gwynn, said it was not an example they will follow.
"You can only spend money once. Who knows what will come with these storms, floods and snow," she said.
"Personally, I don't see mergers - so there's no need to prepare for that eventuality. I don't think the Labour Party will get a majority in the next election so the proposed plans will not go ahead."
A deal between Labour and Plaid Cymru means the merger process has been put on pause until after the May's assembly election.
Leighton Andrews, the minister overseeing the plans, expressed concern over Carmarthenshire's proposal.
He said he would "act to stop any irresponsible spending" in relation to council's reserves.
A Welsh government spokesman added: "The Welsh ministers already have powers to set minimum level of reserves required to be held as part of the authority's budget setting process.
"Additionally, the provisions of the Local Government Act (Wales) prevent improper use of assets before mergers."
Moves to cut the number of councils in Wales to single figures have sparked fears that some local authorities may spend reserves ahead of mergers.