Avraham did threaten to take the land by force as well as offering to pay for it.
See Rashi 23:4 from Medrash Rabba.
גר ותושב אנכי עמכם. גֵּר מֵאֶרֶץ אַחֶרֶת וְנִתְיַשַּׁבְתִּי עִמָּכֶם. וּמִדְרַשׁ אַגָּדָה אִם תִּרְצוּ הֲרֵינִי גֵּר, וְאִם לָאו אֶהְיֶה תּוֹשָׁב וְאֶטְּלֶנָּה מִן הַדִּין, שֶׁאָמַר לִי הַקָּבָּ"ה לְזַרְעֲךָ אֶתֵּן אֶת הָאָרֶץ הַזֹּאת:
גר ותושב אנכי עמכם
I AM A STRANGER AND A SETTLER WITH YOU — A stranger having come from another land, but I have settled down amongst you. A Midrashic explanation is: if you agree to sell me the land then I will regard myself as a stranger and will pay for it, but if not, I shall claim it as a settler and will take it as my legal right, because the Holy One, blessed be He, said to me, (12:7) “Unto thy seed I give this land" (Genesis Rabbah 58:6).
As to the obvious question, how can Avraham threaten to take the land by force when he and his shepherds were the ones who disagreed with Lot and his shepherds about taking the land? See the mefarshei Rashi who explain that Avraham only disagreed with Lot while he had no son, a contingency in the promise of receiving the land. Once he had Yitzchok, he had full rights to the land. That is why Rashi chose to focus on a passuk that included 'לְזַרְעֲךָ , to your children'.
Regarding Avraham's generous offer and hidden threat, remember the sage advise:
"Speak softly, and carry a big stick; you will go far." Theodore Roosevelt.