How to sample from smaller data frame with multiple conditions to a larger data frame?

I have a main dataset df.main with 3 sites and each site has 3 subsites, that were samlpled over three months. I have a separate dataset with some abiotic variables ONLY for a single month df.sample. But for each site, I have three values from the sub-sites. In my original dataset, I need to add the abitoic column. However, for every month, for each sub-site I only want to SAMPLE with replacement from one of the three samples from the sub.site.

set.seed(111)

##Main Data Set
month <- rep(c("Jan","Feb","Mar"), each =9 )
site <- rep(c("1","2","3","1","2","3","1","2","3"), each = 3)
sub.site <- rep(c(1,2,3,1,2,3,1,2,3), time = 3 )

df.main <- data.frame(month, site, sub.site)
  month site sub.site
Jan    1        1
Jan    1        2
Jan    1        3
Jan    2        1
Jan    2        2
...     ..      ..
Mar    3        3

##Sampler Data Set
site <- rep(c(1,2,3), time = 9)
sub.site <- rep(c(1,1,1,2,2,2,3,3,3), each = 3)
abiotic <- rnorm(27,7,1)
df.sample <- data.frame(site, sub.site, abiotic)

  site sub.site  abiotic
 1        1      7.175096
 1        1      8.805868
 1        1      6.783571
 1        2      7.910917
 1        2      7.202307
 1        2      8.404883
 ...
 2        1      7.122915
 2        1      6.152732
 ...
 3        1      7.978232
 3        1      6.870228

##Desired Output in df.main
month site sub.site abiotic
  Jan  1     1      8.805868
  Jan  1     2      7.910917