Martinez revealed he wanted the Australian on loan until the end of the season with a view to a permanent deal.
But he did not have the funds to complete an instant deal in January for McDonald, who has netted 64 goals in 128 games since signing from Motherwell.
"We had wanted Scott on loan with a view to paying the full transfer in the summer.
"I think if they are to sell Scott McDonald then they will want any transfer fee up front but we are simply not in a position to do that.
"I tried to get him in the last window but that was also turned down."