I'm almost certain that your rental contract as well as the document received today state what you're paying/what you owe money for.
Rent is split into net rent and utilities.
Net rent is a fixed amount per month.
Utilities are also a fixed amount per month. They are to be paid either "pauschal" (a fixed amount/lump sum on top of which the landlord cannot charge extra even if the costs generated during the year for utilities are higher than the amount charged to the tenant, and the tenant cannot demand money back even if the costs incurred by the landlord are lower than estimated) or "akonto" (on account). With akonto payments, once a year the landlord compares the amounts charged to the tenants for utilities to the amount he spent on utilities. If he spent less than he charged you, he owes you money back. If he spent more than he charged you, you owe him money.
Your charge of 150.- looks like he is charging on account/akonto for utilities and that he underestimated the amount necessary to pay for costs incurred from July to September.
Check your rental contract to see what he listed under "utilities chargeable to tenant" and compare that to the bill you received today. If he is charging for something which isn't in your contract, you can go to the renter's arbitration office and contest the bill.
E.g. if he charges for yearly garbage disposal, but that isn't listed in the lease, you can contest it. But if he charges for heating according to the lease and it just so happens that heating costs were higher than he expected, you will have to pay.
Note that there are many different costs included in utilities, not just heating. You have the right to ask for copies of proof of cost incurred by your landlord for your building's utilities, and can also ask him how he splits cost. Cost to the individual tenant will differ depending if the landlord charges per number of inhabited sq/m, per number of people living in a flat or if he bases his charges on the tenant's individual, actual consumption.
Are you a member of ASLOCA? If not, I recommend you apply for membership pronto and bring your case to them. They might be able to help. You won't have to use them to go to arbitration, but you can and I recommend doing so.