Safely negotiate a salary for a PhD offer - Germany I have recently received an offer from a group which pursues topics I find very motivating and whose equipment is above average for my field.

The salary offered is 50% of the German TV-L E13 salary, which I think is acceptable by academic standards. However, all other offers which I have received are 65% of TV-L E13, though all of these other groups pursue topics which to me are slightly less interesting. To give you a bit of background, 50% has been the standard for a long time, but since a couple of years ago most groups in my field are deciding to award their PhD employees 65%.

I am very motivated by my particular interests, and had the salary been equal for all offers I would have taken this one without question. Sadly, the situation being as it is, I am quite split. Additionally, the professor made it quite clear that I would also have to get involved in teaching (not all other groups were as forward about this) - which I do not mind in the slightest, but which I think would give cause for extra consideration when calculating a salary. Regarding the professor's interest, he seemed very keen to have me on board, but also very confident I would take his offer. 

I would like to ask whether he could accommodate for 65%. But, since I am yet to decide, I would not like to lose the offer or put great strain on our future relationship if he refused. 

The way I see it, academic culture treats monetary interest with a certain disdain, and I would like to ask you how I could best formulate my request.

FEEDBACK: I have followed the advice offered here, and stated in an email to my prospective PI that I had received better paid offers, and that I would ask him if he could increase the pay, though I am aware of how difficult this could be for him. He took some time to reply (which initially gave me cause to worry), but apparently now he managed to arrange for 65%. I would encourage any of you to follow the advice presented here; and do try to negotiate as long as you do it very respectfully!
One of the mods here is a professor in a german university, I think he is able to add a better answer or at least confirm or discard mine.

Some considerations:


As far as I know, many german PhD students, at least in CS, are actually employed full-time, which I assume means 100% TVL-E. In that sense, both 50% and 65% seem somewhat on the low side to me. At the very least, negotiating for 65% does not seem so unreasonable that it should offend anybody.
However, don't be too disappointed if your negotiations go nowhere. I have heard from multiple people that academic salaries are oftentimes entirely unnegotiable in Germany (though not always)
German PhD student salaries are internationally quite on the high side (also if you take costs of living into account), so even if you cannot negotiate, 50% might still be ok compared internationally. That being said, the question is always if the salary is enough for your standard of living / circumstances. If you have a partner and two kids, maybe 50% simply does not allow you to live. If you are alone and live in a dorm, maybe 50% is plenty.
In germany, some amount of teaching is usually part of the job. That the one professor told you this before is a good thing. However, there is teaching and there is teaching. I have certainly seen groups where teaching load could add up to about 75% of a PhD student's work time. This is obviously terrible (independent of money issues). The hard part is figuring out whether this is the case in your specific group. The best way is to ask one of the current PhD students.
Don't be too afraid that you may annoy your future professor. In my experience, the entire "academics frown upon money issues" is a bit of a show. The same professors who says such things at the same time have no quarrels at all negotiating their own contracts or department budgets. If your future professors gets annoyed by you mentioning that you have a number of better offers, you should likely go somewhere else anyway.


Now, for the actual negotiation, I would follow moriary's remark from the comment - simply mention (friendly and non-threatingly) that you are torn on accepting the offer, as you have counter-offers at 65%. As said, nobody should get annoyed by that. We are all adults, right?