R randomly swap values between two columns in dataframe

Dear stackoverflow community,

I repeated an experiment (rep1 and rep2). For each replicate I have two columns (a, sum) and two rows of the tested subjects that belong together (group AA, BB...). For analysis, I would like to randomly assign the collected data (a and sum) to rep1 and rep2. For this, I was trying to randomly select groups and swap "a" and "sum" of rep1 and rep2. I am trying to repeat the random swapping 100 times, creating 100 datasets for analysis.

I came across unique(df$groups) to specify that the data of each group belongs together. Combined to sample(unique(df$group), 2) it randomly samples, let's say, 2 groups. But I don't know how to swap the data of the replicates of these selected groups. Any help or suggestions are greatly appreciated!!

Here is an example of the data:

group = c("A", "A", "B", "B", "C", "C")
rep1_a = c(2, 8, 5, 5, 4, 6)
rep1_sum = c(10, 10, 10, 10, 10, 10)
rep2_a = c(3, 8, 4, 5, 5, 6)
rep2_sum = c(11, 11, 9, 9, 11, 11)
df = data.frame(group, rep1_a, rep1_sum, rep2_a, rep2_sum)

#    group    rep1_a     rep1_sum     rep2_a   rep2_sum
1     A          2         10          3         11
2     A          8         10          8         11
3     B          5         10          4          9
4     B          5         10          5          9
5     C          4         10          5         11
6     C          6         10          6         11

And here is what it should look like, if out of these 3 groups, the replicates of group A are swapped:

    group     rep1_a    rep1_sum    rep2_a    rep2_sum
1     A          3         11          2         10
2     A          8         11          8         10
3     B          5         10          4          9
4     B          5         10          5          9
5     C          4         10          5         11
6     C          6         10          6         11