What is the Sunnah or the customary practice followed by the Companions and the Tabi'in regarding the place and time of holding the marriage contract (Nikah) when customs differ between the groom's and bride's families, especially if the groom fears falling into what is religiously forbidden due to the delay of the contract?

The general principle is that customs should be observed. If customs conflict, then altruism (īthār) is recommended. Allah praised the Ansar for prioritizing others over themselves. Islamic law encourages leniency in dealings, as in the hadith (definition): "May Allah have mercy upon a man who is lenient when he sells, when he buys, and when he demands his rights." So, either the groom's family should be persuaded to concede, out of consideration for the bride's family, especially if this facilitates a swift completion of the contract, or the bride's family should be asked to concede, out of consideration for the groom's family's custom. If both parties refuse, a middle ground must be reached that combines both customs; otherwise, resorting to lots (qur'ah) is an option.