There seem to be problems with the Ruth/Boaz story.
Scholars disagree as to whether the events described in Ruth 4 relate
to the institution of levirate marriage (also known simply as
“levirate”). Though according to Boaz the marriage will “maintain the
dead man’s name on his inheritance” (Ruth 4:10), the marriage of Ruth
and Boaz does not seem mandated by the law in Deuteronomy, given that
Boaz is not Ruth’s brother-in-law, nor does the genealogy in Ruth 4
credit Ruth’s son to her deceased husband’s line. In some ways, the
transactions described resemble the redemption process for property
outlined in Lev 25:25, where male relatives buy back property an
impoverished kinsman was forced to sell. However, there is no
indication in Leviticus that redemption of a kinsman’s property is in
any way connected to marriage with the kinsman’s widow.
Verses below are from Scripture4all.org's Interlinear program (ISA-2))
In Ruth 4:10, Boaz swears to the elders that he will raise up the name of Mahlon (Ruth's deceased husband).
However, Ruth 4:11-12 sounds like the elders are prophesying to Boaz... Ruth 4:12 "And let thy house be like the house of Pharez (phrtz), whom Tamar bare unto Judah *from-the~seed which he-shall-give Yahweh to~you from the~maiden the~this."
In Ruth 4:18, Boaz is in the genealogy of Perez (phrtz - same spelling as above).
Ruth 4:13 - "...the LORD gave her (Ruth) conception and she bear a son."
Ruth 4:14-16 - It's all about Naomi and how Naomi took the child to her bosom and became foster-mother. Evidently, Ruth died in childbirth.
Ruth 4:17 - "And the women her neighbors gave it a name, saying, 'There is a son born to Naomi'; and they called his name Obed: he [is] the father of Jesse, the father of David."
The last part of that verse - prophesy, or a margin reference? 1Chr. 2:11-12 1Chr. is the only place the lineage appears (outside of Ruth). And Boaz next (and last) appears in 2Chr. as the name of a temple pillar - the left one.
Odd how an 'angel' gave seed to Abraham and when Benjamin was born, Rachel died. Joseph recognized the 'specialness' of Benjamin.
Ruth gets a 'divine seed' and, it appears, dies as well.
It's interesting that Ruth 4:11 says, "...and~do-you! ability in~Ephratah (aphrthe) and~call-you! name in~Beth~Lehem."
Gen. 35:16 - "And~she-is-dying Rachel and~she-is-being-entombed in~way-of Ephrathah (aphrthe) (s)he Beth-Lehem."
Side note: Makes me wonder if Mary died when Jesus was born - and Joseph married someone else named Mary to have further children. Did Joseph then become 'Joseph of Arimethia'? Heh.
Back to the question, however, the way in which the inheritance of the first man may have been affected:
After the first son was born and the child raised to the age of inheritance, all of the money to maintain that land since his birth would be for naught for the husband - as it would all go to Mahlon's namesake.
If Ruth would have had more sons after Mahlon's namesake, those sons would have gotten a part of the 2nd husband's estate - but sometimes too many divisions makes land/assets almost worthless to the inheritors.
Just curious - if Ruth died in childbirth and the child was 'adopted' by Naomi as her own son, does that automatically free Boaz from his promise?
Anyway, just connections that came up in looking deeper into the OP's useful question.