The subject was well discussed and resolved on RpostgreSQL mailing list, linking it if anybody would have same problem: https://groups.google.com/forum/#!topic/rpostgresql-dev/NDc7NfUP6M8
Below the content:
library(RPostgreSQL)
# Loading required package: DBI
c=dbConnect("PostgreSQL")
a <- 1380742793415240
b <- 1380742793415241
dc <-data.frame(a=as.character(a), b=as.character(b))
dbWriteTable(c,"testclosenumberch", dc)
# [1] TRUE
dbGetQuery(c, "select * from testclosenumberch")
# row.names a b
# 1 1 1380742793415240 1380742793415241
dbGetQuery(c, "select a::bigint - b::bigint from testclosenumberch")
# ?column?
# 1 -1