Question Summary:
My wife promised my taxi driver some money, did I need to pay it?

Question Detail:

Asalamualaikum.
Alhamdulillah, whilst we went to do Umrah we also had the pleasure of visiting the area of Khaybar where the battle took place.
Our local driver decided to take us who also arranged, and delivered the original ziarat that comes with the Umrahpackage. We went with him because other taxi drivers would charge us varying and exortionate rates, typical.
Before setting off he said we can pay him whatever amount, and please also bear in mind he said he has visited or taken people to Badr which, is more or less the same distance. By the time we got back to Madinah we gave him 300 riyals = £60 approx for a two hour journey there and back, so 4 hours in total. He was asking for a little more to my surprise because he said "it's the first time he had been to Khaybar". I personally thought he was joking around, but my wife said if we have anything less at the end of our trip we'll pay a little more, and he sort of acknowledged that.
I told my wife we're not going to pay him more because in my opinion 300 riyals is a decent amount, and there was no formal agreement before we set off because he said himself we can pay whatever amount. Our final trip was our trip to the airport, and he didn't ask for the difference there.
Bearing in mind all the above was I obliged to pay him a little more?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The statement of the driver, “Pay me whatever you want” is an invalid specification of the fee for the driver.[1]
When hiring the service of a taxi driver, the fees must be stipulated to avoid a dispute.[2]
If the fees are not stipulated, the Ijārah (hiring) will become defective (فاسد). In such a situation, one is required to pay the standard fee for travelling that distance according to the common practice.[3]
You state that this driver who took you to Khaybar also took you to the airport.
He did not ask for more money. It is clear from his attitude that he accepted the 300 Riyals as that is the standard fee for that distance.
And Allah Ta’āla Knows Best 
Mu’adh Chati
Student Darul Iftaa
Blackburn, England, UK
Checked and Approved by,
Mufti Ebrahim Desai.