In Rus 4:3 Boaz says
And he said to the near kinsman, "Naomi, who has returned from the field of Moab, is selling the portion of the field that belonged to our brother, to Elimelech.
How does Boaz know that this is taking place? If Naomi was poor when returning to Beis Lechem why did she wait months to sell this field? Why is Naomi not present (or at least not speaking) during the sale?
The Malbim on that verse explains that he knew the sale was taking place because she had decided to sell it, and it was publicly announced that the field was on the market.
Alternatively, the Malbim explains that she had told Boaz that she intended to sell it, and he had told her that he would first approach Tov (Ploni Almoni). The Malbim finds support for this in verse 4, where Boaz is seemingly referring to a prior conversation which he is now merely relating to Tov.
I remember reading that Ploni Almoni was Elimelech's brother. Rashi 2:1 says that Boaz was the son of Salmon, a deceased brother of Elimelech and Ploni Almoni was Tov another brother of Elimelech. Rashi 4:1 explains why the name was changed.
a kinsman: Heb. מוֹדַע, a kinsman. He was the son of Elimelech’s brother. Our Sages said: Elimelech and Salmon the father of Boaz and So-and-So the near kinsman were all sons of Nahshon the son of Amminadab, but the merit of their fathers did not avail them when they left the Land to go abroad.
Note laws of inheritance are such that Naomi could not have inherited the property of Elimelech. Since his children were dead, the land would then go back to Elimelech's brother (Ploni Almoni) and then down through that line. In Kesuvos 104, we see that a woman can arrange to have her husband's property sold to pay for the kesuvah. Naomi did not wish to do this for as long as she could avoid it. The implication as well is that Boaz arranged this beforehand so that he could then insist that the redeemer also marry Rus.
Since she was selling the land to cover the kesuvah, the actual redemption and the marriage to Rus was "lifnim mishuras hadin". Only the brothers of the dead husband could be considered in the category of Yibum. As explained by RAMBAM, since none of Elimelech's children were alive, Rus was not subject to the halachos of yibum and could have married anyone she wanted (or who would have her).
There are meforshim who say that Ploni Almoni refused to marry Rus because he knew that there would be a dispute. He considered that she would have to marry a gadol (like Boaz) so that the psak of "Moavi vlo Moaviah" (a female Moabite could marry and be accepted) would be accepted in future generations. Indeed, Dovid was originally rejected by people who did not want to accept that psak and Shmuel Hanavi wrote the book of Rus to emphasize the ruling.
The fact that Naomi waited and did not make this claim until Ruth was recognized as being worthy of marriage (to Boaz) suggests that she was planning this from the start of her return to Eretz Yisrael. The claim that she had on the marriage of her son-in-law Ploni Almoni to Ruth was biased by the land rights too (even though this land was not directly hers). However she could not insist that this land was included with Ruth until the Leverite marriage law had been set aside.
