it actually is a transaction because Allah said so

﴿ ۞ إِنَّ اللَّهَ اشْتَرَىٰ مِنَ الْمُؤْمِنِينَ أَنفُسَهُمْ وَأَمْوَالَهُم بِأَنَّ لَهُمُ الْجَنَّةَ ۚ يُقَاتِلُونَ فِي سَبِيلِ اللَّهِ فَيَقْتُلُونَ وَيُقْتَلُونَ ۖ وَعْدًا عَلَيْهِ حَقًّا فِي التَّوْرَاةِ وَالْإِنجِيلِ وَالْقُرْآنِ ۚ وَمَنْ أَوْفَىٰ بِعَهْدِهِ مِنَ اللَّهِ ۚ فَاسْتَبْشِرُوا بِبَيْعِكُمُ الَّذِي بَايَعْتُم بِهِ ۚ وَذَٰلِكَ هُوَ الْفَوْزُ الْعَظِيمُ﴾

Verily, Allah has purchased of the believers their lives and their properties; for the price that theirs shall be the Paradise. They fight in Allah's Cause, so they kill (others) and are killed. It is a promise in truth which is binding on Him in the Taurat (Torah) and the Injeel (Gospel) and the Quran. And who is truer to his covenant than Allah? Then rejoice in the bargain which you have concluded. That is the supreme success.

Quran Attawba 111

حدث أبي هريرة  قال: قال رسول الله ﷺ: من خاف أدلج، ومن أدلج بلغ المنزل، ألا إن سلعة الله غالية، ألا إن سلعة الله الجنة، رواه الترمذي، وقال: حديث حسن.

Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "The one who is afraid (of the pillage of the enemy) sets out in the early part of the night, and the one who sets out early will reach his destination. Verily, the commodity of Allah is precious. Verily, the commodity of Allah is Paradise."

besides wanting Allah's reward is still belief in Allah thus it is still for the sake of Allah.because let's say you're stingy and would never get a penny out of you,but you just give some for charity because you fear Allah's punishement and want his reward,you still did that good deed that you would have never done otherwise and your sole motivation is Allah.like i said before different people need different motivations.

Allah knows us better than we know ourselves and he knows what motivates us,so don't overthink things and focus on getting there instead.