Sorry, but I do not understand what exactly you meant by the term "travel allowance"

I have worked with different companies in Germany and this is my understanding of so called allowance:
- The company pays your hotel bill and transport (rented car, air ticket, bus ticket etc)
-  With most companies there were standard comepensation paid as you mentioned liek 6, 12 and 24 hrs. 24 hrs it was 24€, basically for your food because dinner you will normally pay with your own pocket. Anyways, that's not much compared to the number of hours you needed to reach the destination and return journey. For e.g. if to and fro needs 6 hrrs and you will be compensated against these 6 hrs. (some companies says 3). You will have 6 hrs+ in your time account. Means some other day you may work 6 hrs less. You can see it like this too..usually CTC/hr of an Engineer is 50-90€, so inderctly you can multiply by 6 which your "travel allowance"
- None of the companies I worked with paid "money" for my travelling hours but yes I always could always comensate travelling hours by working less some other day.
- I am not sure if you really find companies in Germany which will compensate your travelling hrs with cash money. And for 24€ per day I really will not negotiate. First its peanuts compared with your travelling time. Secondly, its policy applicable to everyone, so I do not think they will change it for one person.

Regards
Tony