Saunders, working in France as a pundit for the BBC, left his car in a short stay car park at Birmingham Airport thinking that his side would make an early exit from the competition.
But Wales now face a semi-final showdown on Wednesday with Portugal for a place in the final - and the airport has said they will waive the bill if Wales lift the Euro 2016 trophy.
Saunders said he expected to see his parking charges rise to Â£1,000 following his nation's 3-1 quarter final victory over Belgium on Friday night.
The airport said on Twitter that if Wales win the tournament, it would "waive his parking fee".
Saunders, who won 75 Wales caps and whose former clubs include Liverpool and Aston Villa, also said he was running out of clean clothes.
He was in charge at the Keepmoat from 2011-13 and was responsible for building the 2012-13 League One winning side but left midway through the season to become boss at Championship side Wolverhampton Wanderers.
The airport posted: "We can't help Dean with his clothes situation but if Wales win #EURO2016 we'll waive his parking fee!"